Hellschreiber (idea)

A kind of radio facsimile that was prevalent in the first half of the
20th century, in particular in Germany, but is now used exclusively by
radio amateurs.

Hellschreiber transmits text by dividing each column into 7 pixels, and
transmitting them sequentially, starting at the lowest pixel. A black
pixel is transmitted as a signal, and a white pixel is transmitted as
silence. This takes place at a rate of 122.5 baud (pixels per second).
Since the text was printed on continuous rolls, the number of columns
is indefinite.

The original Hellschreiber machine was a mechanical device, so
therefore it was possible to send "half-pixels" the right
ends of the loops in B, for instance, could be shifted a little, so
as to improve the readability. Any on-signal could in any case last no
shorter than 8 ms, however, both because of having to restrict the
occupied bandwidth on the radio, but also for reasons having to do with
the mechanical makeup of the receiving machinery.

All implementations of Hellschreiber print all received columns twice,
one below the other (but they are not transmitted twice). This is to
compensate for slight timing errors that are often present in the
equipment, and causes the text to slant. The received text can look
like two identical texts coming out one below the other, or a line of
text coming out in the middle, with chopped-off lines above and below.
In either case, at least one whole letter can be read at all times.

Depicting the received signal as shades of gray instead of
monochrome, thereby making it much easier to read weak signals.

Changing to a different font. Here is one mode that is truly
international and independent of character sets: any thing that can be
depicted as markings within a 7 pixels high grid, can be transmitted
over the air.